Method of making iron-chromium alloys



Patented F eb. ;1 7, 1925.

EZEKIEL J. SHAGKELFORD AND WILLIAM B. D. PENN'IMAN, OF BALTIMORE, MARY- l,5Z7,088 I PATENT OFFICE.

LAND, ASSIGNORS TO RADIAC METALS, LTD, OF MASTER'ION, NEW ZEALAND, A

CORPORATION OF NEW ZEALAND.

METHOD OF MAKING IRON- UHROMIUM ALLOYS.

No Drawing.

To all whom it may concern:

Be it known that we, EZEKIEL J. SHACKEL- roan and WILLIAM B. I). PENNIMAN, citizens of the United States, residing at Balti- .more, Stateof Maryland, have invented an Improvement in Methods of Making Iron- Chromium Alloys, of which the following description is a specification.

This invention relates to the production of low carbon alloys of iron and chromium,

either by direct reduction of the chromium out of its ores or compounds into a bath of molten iron or steel, or by removal of the carbon from high carbon chromium iron 7 alloys (ferro chromes) made by the direct arc furnace or ina blast furnace to obtain aproduct with 40 to 65% chromium and 6 to 9% carbon- This high carbon material was then remelted in an electric furnace under a slag containing lime and iron ore, or chrome ore, in order to burn out the carbon. On account of the large amount of carbon relative to the chromium, such methods cause high chromium losses, since chromium is removed from the metal being refined much faster than is carbon. A further disadvantage is that by such methods it is .diflicult and expensive to secure ferro chromes with less than one or two: per cent carbon. Thus, such methods are costly and give low grade products.

Othermethods practiced commercially depend upon substituting the reducing action of carbon by silicon, aluminum, calcium, or silicon carbides, and alloys of silicon and aluminum or of calcium and silicon. These methods. give fairly low carbon ferro chromes, but the cost is high, and objectionable amounts of silicon or aluminum are often leftin the alloy.

In practicing the invention there maybe utilized the various commercial forms of Application filed December 11, 1924. Serial No. 755,302.

carbon as reducing materials, in such ways that there is a minimum contamination of the reduced metal by carbon. Consequently in the subsequent refining stages there is a relatively small amount of carbon to be removed and the chormium losses during refining are correspondingly reduced. Furthermore, by working with a very active refining slag of small volume, the time of refining may be shortened, thus minimizing still further the chromium losses.

Y In order to accomplish. these results in the direct production of chromium from its ores the following features are advantageous, as will be more fully explained hereafter: (1)

a rapid furnace capable of close temperature control; (2) sizing of reducing agents; (3) intimate mixture of ore and reducing material; (4) careful regulation of rate of feeding of the mixture to be smelted relative to the energy supplied the furnace; (5) periodic removal of slag resulting from the reduction of the ore before accumulation of too great a slag Volume; (6) complete removal of the smelting slag before adding the constituents of the refining slag; (7) a' high temperature and a small slag volume during the refining operation; (8) use of less carbon or other reducing agent than that needed for complete reduction of the chromium and iron oxides.

These steps in this process are based upon the following phenomena which have been observed during experiments. When chromium is reduced by carbon-from its commercially available ores into a molten iron or steel bath, slag is formed from the impurities in the ore and carbon; the volume of this slag increases with the amount of ore and reducing agent fed in. The chromium content of the charge decreases and the chromium content of the metal increases so long as the reducing agent is present in the ore mixture being fed in. It has been found, however, that as soon as the reducing agent disappears from the ore mixture, the slag immediately begins totake out chromium and iron from the metal bath underlying it, If the temperature is high and nized by (a) rapid reduction and smelting of chrome ore in the orecarbon mixture, so

' somewhat less thanthe theoretical amount of carbon needed to completely reduce the chromium ore charge.

In the decarbonizing phase it is found that a strongly basic decarbonizing slag as high as possible in chromium oxide is the preferred condition, because such a slag, being already nearly saturated with chromium oxide, tends to rob the bath of but little chromium. We find that the activity of this slag can be so increased by adjusting the temperature that its volume can be materially reduced over that hitherto customary, thus efiecting further savings of chromium in the metal being refined.

Having now-set forth briefly the procedure and the experimental evidence on which it is based, there will now be described the direct reduction process by steps, as applied to the production of stainless steel or iron.

1. Furnace.An arc furnace is preferred of the arcto-molten bath type, with or without bottom electrode. The usual commercial forms of such furnaces may be used, but the preferred form should embodythe following: 1. Two or more electrodes symmetrically disposed over the hearth; 2. Especially good electrode control so as to facilitate temperature adjustment; 3. Lines of furnace and its hearth volume so adjusted relative to the position and size of electrodes that the furnace may be forced in order to facilitate the rapid working which characterizes this process; 4. Refractory lining and bottom chosen to withstand the corrosive action of the slags and the rapid working; 5. Customary facilities for introducing hot metal and for removing slags.

2. The metal bath.-For the production of stainless iron it is preferred to use molten blown metal from a Bessemer converter introduced into the electric furnace. Such metal should not carry more than 05% carbon.

For stainless steelthe' molten metal may be transferred from the converter to the electric furnace before. all its carbon is removed, say at 0.2% to 0.6% carbon.

If molten refined metal is not available from a Bessemer converter or an openhearth furnace the bath can be formed by melting and refining steel scrap in the electric furnace by means well known to the art. In any case, before charging the furnace with metal a limestone charge is first placed on the hearth ofthe furnace.

v3. Reception sZag.-As soon as the molten bath is formed it is covered with a reception slag composed of lime and fiuorspar.

4. Preparation of the mixed ore, flux and carbonaceous reducing materaL-As above stated, there is used finely crushed ore' and reducing material; this applies also to the flux. These ingredients are intimately mixed. I

As a source of chromium there is employed commercial chromite or other chromium-rich materials. We have found 8 to 40 mesh a convenient size for the ore.

For reducing material ground, sized petroleum coke, ordinary coke or anthracite is preferred, although other carbon-rich materials may be used. All fines (i. e., materials passing a 60 mesh sieve) should be carefully eliminated, since it is found that retention. of such fine reducing material causes upheavals and explosions in the rapid reduction process. The preferred size of the carbon particles is 8 to 20 mesh. The use of such sized reducing material has been found of great importance in promoting the rapid reduction of ore which is essential'to success.

For fluxes silica, clay, rutile, ilmenite, fluorspar or lime, singly or in combination are used, according to the nature of the impurities in the ore and carbon reducing 1naterials used. The aim is to secure a slag fluid enough, at the reduction temperatures used, to facilitate ready separation of reduced buttons of metal and the quick removal of the slag itself after disappearance of the'reducing agent from the ore mixture.

5. Charging the ore miwture.As soon as the reception slag is thoroughly fluid. the ore-flux-carbon mixture is introduced by shoveling, or by an ore charging machine. The mixture is spread thinly over the metal bath at such rate that there are never more than a few inches depth of unreduced mixture. A particular object of this method of charging is to minimize contact of carbon particles with the metal bath. The rate of charging is adjusted so as to carefully maintain this condition.

6. Regulation 0 f enemy input 0 f furnace. The electric energy passed into the furnace is adjusted so as to give the maximum rate or reduction without, however, superheating to such an extent as to damage the roof or walls of the furnace. This energy input is naturally different for different furnaces, and has to be ascertained for each furnace by trial. The object to be attained is to feed in ore mixture just as rapidly as it can be reduced and the reduced metal particles incorporated into the metal bath.

7. SZagg'in-g 077'.Generally. it is advantageous to work as follows: After about onehalfthe ore mixture has been fed in, the energy input is slightly increased and the furnace operated until (a) the carbonaceous particles have disappeared: (b) the slag has become fluid: and (e) the chromium content of the metal bath, as shown by quick chemical test or by fracture of a test piece, has reached a maximum, Immediately these conclusion of this stage of the process the metal will contain 10 to 20% chromium and rarely over 0.6% to 1.0% carbon. The time required for this step is usually three to four hours.

9. Demrbon.izing.-Finely ground lime and fluorspar are added to the bath immediately after removal of the last smelting slag, and 'the temperature is raised to a point where these additions melt in ten or fifteen minutes. Finely divided chromium ore is then added gradually by shoveling, or by an ore chargingmachine, distributing it well over the surface of the bath. The high heat is continued with rapid chemical tests for carbon and chromium every ten minutes, until the carbon content has been sufliciently reduced. Usually to hour is sufficient to reduce the carbon below 0.1%.

10. De0widati0n.-In order to remove oxides and gases from the bath, powdered ferro-silico-n or ferro silicon aluminum, or

other deoxidizing alloy, is strewn on the surface of the deoxidation slag. The amount to be addednaturally varies with different heats and is adjusted by quick analysis of the metal bath for silicon or aluminum, or both, and by the appearance of fractured ladle tests. The aim is to prevent introduction of silicon or aluminum into the metal bath and to secure sound metal free from blow holes. 7

1.1. Slaggz'ng 0;? and tapping.1mmediately after the'chemical and fracture tests have shown that the above results have been attained, the decarbonization slag is removed as promptly as possible and the bath covered wit-h powdered lime. The metal is allowed to ,stand in the furnace until at a proper temperature for tapping, after which it is transferred toa bottom-pour ladle and cast-into ingots.

As a specific example of the application of the process the following is a description of the manufacture of stainless iron, without any limitation to the particular proportions or" raw materials cited:

The operation may be conducted in any type of electric furnace which will permit the raising of'the temperature of the contents to the desired degree. electric furnace of about three-quarter ton capacity, supplied with single phase alternating current, and having two depending electrodes arcing to the slag, the furnace lined with chrome brick free from carbon over the bottom and sides up to and above We usedan the slag line; above the slag line, and for the roof was used a high grade refractory brick that was capable of withstanding the temperature. The area ofthe furnace at the slag line was approximatelynine square feet. In operating, molten iron or steel was poured in the furnace, and a basic slag melted thereon.

To a molten bath of 700 lbs. or iron carrying about 0.02% carbon there were added lbs. lime and 15 lbs. fluorspar. The furnace at this stage was operated at 120 volts and 1200 amperes. After 15 minutes the lime and fluorspar were melted, form- 'ing the reception slag. The voltage was maintained at 120 and the amperes at 1500, and approximately one-half the ore-carbonfiux mixture charged in by shoveling as already described. There were used 550 lbs. chromite ore, 117 lbs. coke carrying 12% ash, and 12 lbs. fluorspar. After 1 hours, all the ore was in, and after another 1% hours, the first metal sample was taken for test. The electric energy supplied to the furnace was at the approximate rate of from 600 to 700 watts per pound of chrome ore charged, and at the approximate rate of from 18000 to 22000 watts per square foot of slag surface. The slag was then fluid and showed absence of coke granules; the metal bath showed 8.12% chromium. The slag was removed and the charging ofthe second portion of the ore-fiux-carbon mixture begun, using the same current and voltage as on the first half. At the end of 2 hours the slag appearance and the chromium content of the bath indicated that the slag should be removed. The decarbonizing slag was then formed by adding 15 lbs. lime and 10 lbs. fluorspar and melting by the application of 120 volts and 2000 am-,

peres for 15- minutes. Continuing with the same current conditions there'were then added gradually lbs. of chromium ore. The electric energy supplied during the decarbonizing stage was at the'approximate rate of 23000 to 28000 watts per square foot of slag surface. After 30 minutes, metal samples were taken every ten minutes for quick carbon determination. At the end of 1% hours the carbon was 0.10% and the chromium 12.53%. Ten lbs. of powdered 50% ferrosilicon were then added and the slag removed after ten minutes. The current was shut off and powdered lime added as with, of course, suitable increases in the chromium ore, coke and flux additions, and decreases in the relative amount of low car- A..l adle bon in the initial bath. In decarbonizing, the ratio of slag volume to chromium content of the bath is kept substantially the same as in making stainless iron or stainless steel.

In decarbonizing ferrochrome made by heretofore known processes, the same decarbonizing principlesare followed. It is, however, much preferable to use ferrochrome made by the process described herein since, as stated, it is initially much lower in carbon than that produced by the usual carbon reduction methods, and hence can be refined more economically to a low carbon product.

Having described the process and invention, what is claimed and desired to patent 1s:

1. The method of making iron-chromium alloys which comprises successively reducing quantities of chrome ore in contact with a slag over a molten body of iron or steel, removing the slag periodically and supplying new slag material, of the same reducing character, and oxidizing the carbon from the resulting chromium alloy by supplying a non-carbon charge after removing the reducing slag until the carbon is lowered to a permissible content.

2. The method of making iron-chromium alloys which comprises successively charging and reducing quantities of chrome ore in contact with a slag over a molten body of iron or steel, removing the slag between F the charges of chrome ore, oxidizing the carbon from the resulting chromium by a slag and chrome ore at a higher temperature until the carbon is lowered to a permissible content, and then deoxidizing the chromium alloy. 7

The method of making iron-chromium alloys which comprises melting a slag containing lime and fiuorspar over a body of molten iron or steel, periodically charging a mixture of chrome ore. carbon, and fluorspar upon said slag, reducing said chrome ore and periodically removing slag, oxidizing the carbon from the resulting alloy by a slag and chrome ore at a higher temperature until the carbon is lowered to a permissible content, and deoxidizing the resulting alloy.

4. The method of making iron-chromium alloys which comprises melting a slag containing fluxing and refining agents, upon a body of molten iron or steel, periodically charging a, mixture ofvchrome ore, carbon and fluorspar upon the surface of the slag, reducing said chrome ore, removing slag between the successive charges, of chrome ore mixture, adding slag materials, raising the temperature of the mass with the successive additions of chrome ore mixture, oxidizing the carbon from the resulting chromium alloy by fresh slag material and chrome ore at a higher temperature, and deoxidizingthe resulting alloy.

5. The method of making iron-chromium alloys which comprises successively reducing quantities of chrome ore associated with a reducing agent and slag material, over a molten body of iron or steel, supplying electric energy at the approximate rate of 500 to 700 watts per pound of chrome ore charged, depending upon the size of the furnace and the charge therein, removing the slag periodically and supplying new slag material, oxidizing the carbon in the alloy formed at an increased temerature until the carbon is reduced to a permissible quantity, and withdrawing the alloy from creased temperature until the carbon is re- I duced to a permissible content, and with drawing the alloy.

7. The method of making iron-chromium alloys which comprises successively reducingquan-tities of chrome ore associated with a reducing agent and slag material over a molten body of iron or steel, supplying electric energy at ,the approximate rate of 500 to 700 watts per pound of chrome ore charged, removing the slag periodically and charging additional quantities of chrome ore mixture periodically, maintaining the supply of electric energy to the furnace during the reduction period, withdrawing the slag after suificient chrome ore has been reduced and charging new slag material. oxidizing the carbon in the alloy by the addition of chrome ore and a very considerable increase in the supply of electric energy to the furnace. thereby raising the temperature, and continuing the high tem' perature treatment until the carbon has been reduced to a permissible content.

8. The method of reducing chrome ore to chromium or chrome alloy, and making iron-chromium alloys. which comprises supplying a body of molten iron or steel with slag material, charging thereupon a quantity of chrome ore associated with a reducing agent and a flux, supplying electric energy at the approximate rate of 500 to 700 watts per pound of chromium charged, removing the slag periodically and charging additional quantities of chrome ore-mixture and slag material periodically, the electric energy supply being maintained at approximately 18000 to 22000 watts per square foot of slag surface covered by chrome ore mixture, removing the slag after the reduction of the chrome ore, supplying new slag material, and oxidizing the carbon in the resulting alloy by additions of chrome ore and increasing the supply of electric energy to the furnace, until the carbon is reduced to a permissible content.

9. The method of reducing chrome ore and of making iron-chromium alloys, which comprises supplying a body of molten iron or steel with a quantity of chrome ore associated with a reducing agent and a flux, supplying electric energy as heat at the approximate rate of 18000 to 22000 watts per square foot of slag surface until the chrome ore is reduced and a sufiicient quantity of chromium has been alloyed with the iron or steel, periodically changing the slag and renewing the supply of chrome ore mixture, and oxidizingthe carbon from the resulting alloy by reacting thereon with an oxidizing slag and chrome ore with an increased supply of electric energy until the carbon is reduced to a permissible content.

10. The method of making iron-chromium alloys, which comprises supplying to a body of molten iron-containing metal a slag material, charging thereupon a quantity of chrome ore associated with a reducing agent, supplying electric energy at the approximate rate of 18000 to 22000 watts per square foot of slag surface, periodically replenishing the supply of chrome ore and reducing agent and slag materials, oxidizing the carbon in the resulting alloy by reacting thereupon with an oxidizing slag and chrome ore with an electric energy supply of ap roximately 25000 watts per square foot of s ag surface, until the carbon is reduced to a permissible content.

11. The method of making iron-chromium alloys, which comprises supplying to a body of molten iron-containing metal a slag ma terial, charging thereupon a quantity of chrome ore associated with a reducing agent,

supplying electric energy at the approximate rateof 18000 to 22000 watts per square foot of slag surface, periodically replenishing the supply of chrome ore and reducing agent and slag materials, oxidizing the carbon in the resulting alloy by reacting thereupon with an oxidizing slag and chrome ore with an electric energy supply of approximately 23000 to 28000 watts per square foot of slag surface, until the carbon is reduced to a per missible content. 7

12. In the method of making iron-chromium allows, thestep of decarbonizing the same which com rises covering said alloy with an oxidizing slag and supplying chrome ore thereto, and heating the same i by supplying electric energy as ieat at the approximate rate of 23000 to 28000 watts per squarefoot of slag surface until the carbon is reduced to a permissible content. 13. In 'the method of making iron-chromium alloys, the step of decarbonizing the same which comprises covering said alloy with a slag rich in chromium oxid and poor in iron oxid, supplying chrome ore thereto, and heating the same by supplying electric energy as heat at the approximate rate of 23000 to 28000 watts per square foot of slag surface until the carbon is reduced to a permissible content.

14. The method of reducing chrome ore and of making iron-chromium alloys, which comprises supplying a body of molteniron or steel with a quantity of chrome ore associated with a reducing agent and 'a flux, supplying electric energy as heat at the approximate rate of 18000 to 22000 watts per square foot of slag surface until the chrome ore is reduced and a suflicient quantity of chromium has been alloyed with the iron or steel, periodically changing the slag and renewing the supply of chrome ore mixture, and oxidizing the carbon from the resulting alloy by reacting thereon with an oxidizing slagand chrome ore with an increased supply of electric energy until the carbon is of slag surface, periodically replenishing the supply of chrome ore and reducing agent and slag materials, oxidizing the carbon in the resulting alloy by reacting thereupon with an oxidizing slag and chrome ore with an electric energy supply of approximately 25000 watts per square foot of slag surface, until the carbon is reduced to a permissible content and then deoxidizing the alloy.

16. The method of making iron-chromium alloys, which comprises supplying to a body of molten iron-containing metal a slag material, charging thereupon a quantity of chrome ore associated with a reducing agent, supplying electric energy at the approximate rate of 18000 to 22000 watts per square foot of slag surface, periodically replenishing the supply of chrome ore and reducin agent and slag materials, oxidizing the carbon in the resulting alloy by reacting thereupon with an oxidizing slag and chrome ore with an elec-. tric energy supply of approximately 23000 to 28000 watts per square foot of slag surface, lu'ntil the carbon is reduced to'a permissible content and then-deoxidizing the alloy.

17. In the method of making i'r0n-chrom ium alloys, the step of decarbonizing the same which comprises covering said alloy with an oxidizing slag and supplying chrome ore thereto, and heating the same by supplying electric energy as heatatthe approximate rate of 23000 to 28000 watts per square foot of slag; surface until the carbon is reduced to a permissible content and then deoxidizing the alloy.

18. In the method of making iron-chromium alloys, the step of decarbonizing the Same which comprises covering said alloy with a slag rich in chromium oxid and poor in iron oxid, supplying chrome ore thereto, and heating the same by supplying electric energy as heat at the approximate rate of 23000 to 28000 Watts per square foot of slag surface until the carbon is reduced to a permissible content and then deoxidizing the alloy.

EZEKIEL J. SHAGKELFORD. ILLIAM B. D. PENNIMAN 

